Despite advances in immunosuppression and routine PRA screening prior to transplantation, acute and chronic antibody-mediated rejection (AMR) undermine long-term survival of transplanted organs. The success of therapies targeting AMR in transplant patients is limited by the incomplete understanding of the mechanisms underlying DSA and autoantibody generation and functions. Previous studies of humoral responses following transplantation were impeded by the lack of reliable assays to detect allo- and autoreactive B cells and by the paucity of physiologically relevant animal models of AMR. Production of pathogenic allo- and autoantibodies requires interactions between B cells and activated helper CD4 T cells. The T cell repertoire of many humans includes alloreactive memory T cells that are resistant to immunosuppression or costimulatory blockade. During the previous funding cycle, we demonstrated that memory CD4 T cells induce superior donor-specific alloantibody (DSA) responses compared to primary effector CD4 T cells. We have developed physiologically and clinically relevant mouse models of acute and chronic AMR to investigate the mechanisms of allo- and autoantibody generation after kidney transplantation. Our preliminary data show that donor-reactive memory CD4 T cells induce robust DSA and autoantibody responses in renal allograft recipients resulting in acute AMR. In contrast to de novo DSA, autoantibody generation is not prevented by anti-CD20 antibody treatment and is associated with chronic graft tissue injury. The goal of this project is to determine helper signals required for generation of pathogenic allo- and autoantibodies and to use this information to inhibit AMR in sensitized renal allograft recipients. We hypothesize that the molecular requirements for allo- and autoantibody production following renal transplantation and the specificity and pathogenicity of resulting antibody are determined by the functional phenotype of helper CD4 T cells and by the intensity of post-transplant inflammation. We will test this hypothesis in three Specific Aims: Aim 1. To determine helper requirements for the production and maintenance of donor MHC class I- and MHC class II-specific DSA and collagen IV- and fibronectin-specific autoantibodies in renal transplant recipients. Aim 2. To test the role of BAFF/APRIL cytokine network during the generation of pathogenic allo- and autoantibodies in renal allograft recipients. Aim 3. To test the effects of post-transplant inflammation on the generation of allo- and autoantibodies following renal transplantation.